Variable resistor



Aug. 9, 1938. H. e. RICHTER I VARIABLE RESISTOR Fiied July 12, 1934 2Shets-Sheet 1 .86 n w 2 5 w n m INVEN:I'OR Henr G. Richler BY f 9A'ITQBNEYS Aug. 9, 1938. H. G. RICHTER 2,125,954

I VARIABLE RESISTOR Filed Ju1 12, 1954 2 Sheets-Sheet 2 INVENTOR BY 5 zI a ATTORNEYS Patented Aug. 9, 1938 UNITED, STATES,

- PATENT OFFICE- 27 Claims.

This invention relates to variable resistance elements, and moreparticularly to high resistance elements commonly employed for volumecontrol purposes.

The object of the present invention is to generally improve variableresistance units, especially such units in which the resistance elementis a thin film of carbonaceous, graphitic, or other suitable resistancematerial. More particular objects of my invention reside in theprovision of a unit of this class which will be compact yet utilize allof the resistance material at the maximum radius permitted by theover-all dimension of the unit; which will contact the full width of thefilm by a line contact, thereby avoiding short-circuiting of substantialfilm area; which will be characterized by parallel resistance paths ofequal length, thereby avoiding unequal loading of parts of the film; andthe calibration or resistance value of which will not be modified in usedue to play or wobble of the control shaft of the unit. These objects Ifulfil, generally, by

coating the resistance filmon a ring-like insulation base having acylindrical periphery, which construction results in further importantmanufacturing advantages, such as the ease of uniformly or similarlycoating a large quantity of resistance elements, and the ease ofcalibrating the elements before inserting the same in the completeunits. I

A further object of my invention is to so construct the unit that itmaybe made up of simple elements economical to manufacture and assemble,and resulting in a rugged durable unit. Still further objects centerabout the soldering lugs for connecting the unit in circuit, and are tosupport the lugs in desired position and to connect the same to theresistance element and to the movable contact arm without appreciablyincreasing the bulk, complexity, or expense of the unit; and to makepossible the addition, when desired, of a lug connected to or tappingthe resistance film at any desired point while positioning said lug at afixed point on the unit.

Still another object of my invention resides in the provision of astandard unit having a casing open at one. end with which there may beinterchangeably used, at the option of the manufacturer, either a simpleclosure plate or a power switch automatically operated by the resistor,or one or more tandem units all operated in unison with the main unit.Considerable difllculty has heretofore been caused the manufacturer bytandem units, especially in taking care of replacement orders, becausethe tandem units are bought in accordance with a variety ofspecifications, and the types which must be kept on hand for replacementmay run to extremely large numbers.

In accordance with my invention, only the individual. units need bestandardized and kept on 5 hand, for the varied combinations ofdifierent units may be assembled as desired when ordered. Theconstruction is valuable in the field, because a service man mayassemble a single needed unit for a repair job, and is valuableeven forpro- 10 duction purposes because separate standardized units may beassembled at the factory in accordance with the orders received. Thetandem units are themselves open at one end and may be closed by aclosure plate or a power switch or 16 another tandem unit, just as inthe case of the main unit, thus producing an assembly which isexceedingly flexible in meeting varied commercial requirements.

To the accomplishment of the foregoing and such other objects as willhereinafter appear, my invention consists in the variable resistorelements and their relation one to the other, as hereinafter are moreparticularly described in the specification and sought to be defined inthe claims. The specification is accompanied by 1 drawings, in which:

Fig. 1 is a rear elevation of a variable resistor embodying features ofmy invention, the interior of the resistor being exposed by removal ofthe closure plate;

. Fig. 2 is a section taken in the plane oi the line 2--2 of Fig. 1;

Fig. 3 is a section taken in the plane of the line 3'-3 of Fig. 1'; v v

Fig. 4 is a section through a soldering lug, and is taken in the planeof the line 4-4 of Fig. 1;

Fig. 5 is a section illustrating the location of an additional solderinglug for intermediately tapping the resistor, and is taken in the planeof the line 55 of Fig. 1;

Fig. 6 is a similar section in place;

Fig. 7 shows the resistance element;

Fig. 8 shows a resistance element provided with an intermediate tap;

Fig. 9 shows a power switch assembly which may be applied to theresistance unit; v

Fig. 10 is a side elevation of the resistor and switch assembly;

Fig. 11 is a side elevation of a resistor and tandem resistorpreparatory to assembly of the same;

Fig. 12 is a front sistor;

with the added lug elevation of the tandem re- Fig. 13 shows the contactshoe of the resistor; and

Fig. 14 shows a modified thrust bearing for the control shaft.

Referring to the drawings and more particularly to Fig. '7 thereof, theresistance element comprises an insulation ring or base 12 the outerperiphery of which is coated with a resistance film M. The ring i2 maybe made of any suitable insulation material, preferably a moldedsynthetic resin or condensation product. The periphery of the ring ispreferably cylindrical, and the ring may most simply be made rectangularin cross-section. The resistance film l4 may be made of known resistancepaints preferably of the carbonaceous or graphitic type. As is moreparticularly described in my copending application Serial No. 734,811,filed concurrently herewith, a large number of the rings l2 may bestacked on a mandrel and simultaneously and uniformly sprayed with thepaint. In the said application. I further disclose how the element maybe given a uniform or a tapered resistance variation, as may be requiredfor any particular purpose. The film is interrupted or ground away atl8, and the breach I8 is preferably aligned with a keywav i8 on theinner wall of the ring. The ring is preferably made with sufficientradial thickness to make the same rigid and to permit mounting holes 20and an assembly hole 22 to be drilled therethrough in an axialdirection. At the same time there is a large hole left at the centerwhich is utilized as is subsequently described.

Referring now to Figs. 1 through 5, the resistance element I2 is mountedwithin and housed by a casing 24. This casing is preferably a singlepiece of molded insulation and comprises a cylindrical wall 25 closed atone end by a front wall 28, the other or rear end of the casing beingleft open. A threaded bushing 80 of conventional type is secured tocasing 28 in any desired manner. In the present case the end 32 of'thebushing is fluted and driven into the casing with a force fit to preventrotation of the bushing, and lips 88 are thereafter expanded or stakedoutwardly.

The resistance element I2 is seated around and located by a ledge 36arranged concentrically with bushing 80, and is held in place by screws38 passing through the element and threaded into the casing.

The resistance film is yieldably engaged by a slidable contactoscillated by a control shaft 48 carried in bushing 30. Control shaft 48is squared at its inner end 2, and the squared end has permanentlymoullte :raereon a metallic thrust washer ll. an insulatim washer 46,the dished hub 48 of radial contact arm 50, an insulation washer '52,and, finally, a stop arm 54. The shaft end 82 is riveted. or may bestaked outwardly, as by the four punches 58 indicated in Fig. 1. Shaftwith its contact arm assembly is slid throughbushing 80 and is held inplace by a split ring 88 set'into a suitably located groove 80 on thecontrol shaft.

The contact arm of the resistor comprises not only the hub portion 48and radial portion 58 previously referred to, but also a yieldablearcuate portion 62 best shown in Fig. 1. Radial portion 80 overlies theface of the resistance element 12, and its outer end 64 is bent inwardlyor forwardly in generally parallel relation to but spaced fr mresistance film i4. Arc-date portion 82 is unmrm -':*:;1; .2: ."n-retaicurved to conform to the curvature of the resistance element. It issecured at one end to the radial contact arm by a rivet B6 and by amating indentation and recess at 68 taking the place of a second rivet.Strap I32 is preferably substantial in length and in the present casefar exceeds a quadrant, thereby affording a yieldable and non-chatteringengagement between the contact shoe l0 and the resistance element.

Contact shoe 78 is exceedingly simple in form, as is best shown in Fig.13, it comprising merely a rectangular surface having at each endupwardly bent ears 12 which are curved in outline to act as fulcrum pinsfor the shoe. The surface of the shoe may be plane, thus affordingtangential or line contact with the resistance film. It is preferablymade of a non-oxidizable metal. It is supported between the resistanceelement and the contact arm and is moved with the contact arm by reasonof seats or recesses 14 which receive the ears I2. It will be manifest.from inspection of Fig. 2, that the contact shoe 10 may oscillatesidewardly to seat itself across the full width of the film regardlessof play or Wobbllng of the control shaft. Also, that the shoe may have awidth equal to the full width of the resistance film, the front wall ofthe casing being recessed outside the ring i 2, as is clearly shown inFig. 3.

Appropriate soldering lugs are provided for connecting the resistor incircuit. In the present :r.

case, lugs 16 and 18 are provided for connection to the ends of theresistance film, although it will be understood that either lug may beused alone when desired. Lugs 16 and 18 pass through the .outer wall 28of the casing and are clamped between the resistance element i2 and theend wall 28 of the casing, as is best shown in Fig. 4.

The inner end of the lug is preferably reversely bent, as shown at 80,and wall 28 is preferably cut away to receive the. lug and to preventlateral displacement thereof. Referring now to Fig. 7, a highlyconductive paint is applied from the resistance film around the face ofthe element, as is shown at 82, for contact with the soldering lugs 18and 18. Reverting to Fig. 4, the end 80 of lug I6 is recessedslightly'less than its thickness so as to insure good contact betweenthe conductive paint 82 and the lug. To further promote good contact atthis point, the screws 38 which secure the resistance element within thecasing, are not located diametrically opposite one another, but insteadare preferably disposed nearer the soldering lugs, as is clearly evidentfrom inspection of Fig. 1. The contact between the resistance elementand the soldering lugs causes a slight bias of the resistance elementrelative to the control shaft, as is indicated in Fig. 2, but this is nodisadvantage, and, on the other hand, is actually of advantage becauseit results in a slight oscillation of the contact shoe relative to thecontact arm as the unit is operated, and this oscillation prevents theaccretion of undisturbed dirt or oxide between the contact shoe and thecontact arm.

Connection to the contact arm is made by the center lug which passesthrough the casing and beneath the resistance element, as is best shownin Fig. 2. This lug does not, however. contact the resistance fllm andis accordingly preferably located at the breech I6 between the ends ofthe resistance film. The lug is bent upwardly or rearwardly at 88 withinthe keyway l8 of the resistance element. The inner end of the lug bearsdirectly against the contact arm and is preferably formed into a ring88, best shown in y 2,120,904 Figs. 2 and 3, which ring surroundsinsulation washer 40 and bears against the forward side of the hub 40 ofthe contact arm. If desired, teats 00 may be formed on ring 00 toimprove the electrical contact. The lug is. of course, made of materialsuiliciently resilient to provide a yieldable or spring engagement withthe hub of the contact arm. The contact arm is. of course, preferablyinsulated from the control shaft, as has heretofore been described. Theconnection between the lug 04 and the contact arm 00 does not add to theaxial dimension of the casing, because it takes place inside the ringI2.

The unit shown in Figs. 1 through 5 is arranged to receive an additionalsoldering lug for intermediately tapping the resistance element. Forthis purpose the casing 24 is apertured and provided with a recess 92.best shown in Figs. 1 and 5 and corresponding to those provided at thelugs I8 and I8. but the recess 92 is disposed diametrically opposite thelug 84. When an intermediate tap is sought it is merely necessary topreliminarily insert a soldering lug 04 in recess 92 before securing theresistance element I2 in place, as is shown in Fi 6. While the tap isdisposed at the center of the resistance element it is not necessarythat the electrical connection be made at the same point. and it ispossible to change the tapping point without shifting the physicallocation of the lu This is accomplished as shown in Fig. 8. in which itwill be noted that conductive paint is extended from the resistance filmat the tapping point around to the face of the element and is thencontinued along the faceof the element to a point. opposite the keywayI8 or registering with the fourth lug 94.

The open or rear end of casing 24 is closed by a heavy gauge sheet metalclosure disc I00. This disc has a diameter larger than the inner butsmaller than the outer diameter of the casing, and the edge of thecasing is stepped or recessed at I02 to receive the disc I00. The discis provided with a locatin projection I04, and the .edge of the casingis cut away to receive the projection I04. This insures registration ofthe disc with respect to the casing. The casing is held in place by asingle assembly screw I06 which passes through the front wall 28 of thecasing. then through the hole 22 of the resistance element. and isthreaded in a mating hole I00 in the closure disc. This screw is soplaced on the unit that it does not interfere with the oscilla tion ofthe contact arm. which possibility is an advantage of the illustratedcontact arm, while the use of a single through screw for a strong, rigidassembly is itself an improvement over prior constructions.

The closure disc is struck inwardly to provide a motion limiting stoplug IIO (see Figs. 3 and 5) a which cooperates with the stop arm 04heretofore referred to as secured to the inner end of the control shaft40. The end II2 of stop arm 54 is stepped rearwardly. as is best shownin Fig. 2, in order to brin the same close to the disc I00 I and inalignment with the stop lug IIO while at the same time affording ampleclearance between the stop lug H0 and the remaining mechanism at the endof the control shaft. A locating lug H4 is bent inwardly at the oppositeend of stop arm 54 and is received in a mating recess in the insulationwasher 52. It will be understood that with the herein-describedconstruction the various parts are necessarily kept in correctregistration and relation during assembly of the unit.

In many cases the resistor, particularly when used as a volume control,is combined with a power s witch for controlling the energization of thecomplete radio receiver, phonograph, or other device in which theresistor is used. The present unit is readily modified by substituting apower switch assembly for the closure plate I00. Thus, referring toFigs. 9 and 10, I provide an insulation switch casing I20 closed at itsforward face by a metal disc I22 dimensioned like closure disc I00 andsimilarly provided with a projection I04,1a threaded opening I00, and anappropriate stop lug "0'. Connections are made to the switch throughlugs I24. The switch is preferably of the toggle variety and iscontrolled by a rocker I25 to which are secured bearing surfaces I20which project through a window I00 in disc I22. The bearing fingers I20lie in the path of movement of stop arm 54, and the parts are soarranged that when the stop arm reaches its end position the switch isturned oil, this being the condition of the parts shown in Fig. 9 inwhich stop arm 54 is indicated in broken lines. It will be understoodthat when the resistor is turned from its end position the switch istripped during the first part of the movement. The switch is held inassembly with casing I02 (or casing 20) by means of assembly screw I00,received in threaded holes I00. The screw does not interfere with theswitch mechanism, not only because of its location, but also because ofits shortness, for it terminates just inside the plate I22.

For some purposes it is necessary to provide Such units have heretoforeproved troublesome to stock because of the variety of combinations ofresistance and taper values for the individual units in addition to thevariations commonly met with for single units. These problems areminimized by a further feature of my invention in accordance with whichmultiple resistor units are provided by adding one or more tandem unitsto a standard resistor. Thus, referring to Figs. 11 and 12,.the resistorI32 is a standard unit such as has heretofore been described. Instead ofclosing the back of the unit with-either a closure disc or a powerswitch, I may add a tandem unit I04. This in turn may be open at itsback face and adapted to be closed by a closure disc or a power switchunit. The back edge of the tandem unit I04 is preferably made exactlylike the back edge of the standard unit I02, so that the usual closuredisc I00, or the switch unit of Figs. 9 and 10, or

still another tandem unit may be used to close like the standard casingexcept that its forward edge is stepped or shouldered at I00, andprovided with a locating projection I00, thus adapting the unit to matesnugly with the unit I02. In other words, the front end of unit I04 isshaped like and replaces the closure plate I00. The assembly screw I40is approximately twice as long as before, and passes through the pair ofunits from end to end. i

The internal mechanism of the tandem unit I04 is exactly like thestandard unit I02. The mechanisms differ slightly externally due to thefact that whereas the control shaft 40 of the standard unit projectswell out of the casing, the

. is taken in section like Fig. 3.

a drive disc I48. Disc- I46 is slotted at 8, the waste metal beingbentoutwardly to form bearing-surfaces I50 so located and dimensioned as toreceive the stop arm 54 of the main-unit I32.

It will thus be understood that when the units are. nested together,stop arm. is no longer used as a stop arm but merely as vcouplingmechanism to rotate the succeeding unit. However, the stop arm of thetandem unit i then assumes the motion limiting function or/and the]power switch operating function. It will also be under,-- stood that -asmany tandem units =may be added as desired, and in each case the unitisrotated by the stoparm of the preceding unit while the stop arm of thefinal unit is used for motion limiting or/and switch control functions.

In accordance with a further feature of my invention, I make possiblethe-elimination of'a split ring for holding the control shaft in place.Referring to Fig. 3, it is customary to use a split ring 58 in a groove60 to hold the control shaft against axial movement. This arrangement issimple and convenient except for the tendency on the part of somemanufacturers using these units to specify a preferred length for thethreaded bushing 30. in many cases differ only by'a small'fraction of aninch, yet it is necessary for the, manufacturer of the resistor toprovide different shafts havin the groove 60 locateddilferently by acorresponding amount.

I overcome this cilllculty and at the same time provide a morefrictionless type of hearing by an arrangement illustrated in Fig. 14,which figure Referring thereto, the shaft 10' is like shaft ill but isdevoid of the groove to and split ring 58. The 'closure plate I is likeclosure plate I00 but is provided with a hole or seat M0 for a singleball'bearing I62 which is interposed between theclosure' plate and theend of the control shaft. The end of the control shaft is preferably notrecessed to receive ball 162, but instead is left fiat. This is done todispense with the" necessity for accurate alignment of the seat I60 withthe shaft axis, or,

in other words, to greatly increase the-manufacturing tolerancesinvolved. Incidentally, the squared end 42' of shaft 40f may be.securedto the stop arm 54' slightly differently than heretofore-described, asby using a round end for the shaft which is passed through a square holein the stop arm and which end projects beyond the stop arm and is thenriveted downwardly by a high-speed riveter which causes the metal toflow outwardly into the square hole. To assemble the unit-it is simplynecessary to place the ball I62 on closure plate I00 while holding theplate horizontally, and to then place'the unit downwardly over theclosure plate, after which the assembly screw I is screwed in place.

It is believed that the mode of constructing and using, as well as themany advantages of my improved variable resistor, will be apparent fromthe foregoing detailed description thereof. A

high resistance value of uniform or tapered characteristic is readilyobtained because the resistance is merely a film. This film is appliedto and backed by a sturdy rigid insulation base. The use of acylindrical ring facilitates manufacture and calibration of the unitsand is of advantage in operation because the resistance paths are all ofequal length-and therefore uniformly loaded. Variations fromconcentricity in the unit, or wear and consequent play of the controlshaft do not affect the resistance and calibration- These. preferredlengths j 'of the unit. The contact shoe makes a straight line contactwith the resistance film, thereby avoiding short-'circuiting' ofsubstantial film area, which is of particular value in taperedresistance units. The .unit is compactin' dimension because all of theresistance film is used at nearly the maximum radius of the unit. Asmooth, uniformly light, and non-chattering contact is provided betweenthe wiper shoe and the resistance film, and the shoe is freeto seatitself across the width of the film. The unit is readily adapted,to'meet varied requirements because of the interchangeability of theclosure disc, the power switch, and the tandem unit. An intermediatelytapped soldering lug 'may be provided when desired, and the tappingpoint may be varied without physically shifting the lug. More than oneintermediate lug may be used if the casing is modified to receive thesame. The unit is simple, compact, inexpensive, and sturdy.

A metal casing may be used if lined with sheet lnsulationi The solderinglugs may be secured directly to the ring, as by riveting through thering. It will therefore be apparent that while I have shown anddescribed nu; invention in preferred forms, many changes andmodifications may be made inthe structures disclosed, without departingfrom the spiritof the invention, defined-in thefollowlng claims. -Iclaim:

1. A'variabie resistor comprising a rigid preformed closed ring-ofinsulation material having a cylindrical outer surface, the radialthickness of said ring being largenenough to-give the ring structuralrigidity yet small enough to leave a substantial space at the center ofthe ring, a film of resistance material coated directly on saidcylindrical outer surface. a control shaft, and

slidable contact means insulatedly secured to said shaft and slidablybearing directly against the resistance him. and connection means tosaid contact means, said connection means being disposed within saidring-and including a metallic ring and a wiper in contact therewith,said ring and wiper being insulated from the control shaft.

2. A variable resistor comprising a rigid pre-.

formed closed ring of insulation material having a cylindrical outersurface, the radial thickness of said ring being large enough to givethe ring structural rigidity yet small enough to leave a substantialspace at the center of the ring, a film of resistance material coateddirectly on said cylindrical outer surface, a control shaft concentricwith the ring, a yieldable contact arm insulatedly secured to saidshaft, and connection means to said arm, said connection means beingdisposed within said ring and including a metallic ring and a wiper incontact therewith, said ring and wiper being insulated from the controlshaft. and a contact shoe self-adjustably mounted on said arm andslidable directly on the resistance film.

3. A variable resistor comprising a rigid preformed closed ring ofinsulation material, a film of resistance material coated directly onthe outer peripheryof the ring, the radial thickness of said ring beinglarge enough to give the rin structural rigidity yet small enough toleave a substantial space at the center of the ring, a control shaftconcentric with the ring, a contact arm insulatedly secured to saidshaft, and connection means to said arm,'said connection means beingdisposed within said ring and including a metallic ring and a wiper incontact therewith, said ring and wiper being insulated from the controlshaft, said contact arm comprising a radial portion at one face of thering, and a resilient arouate portion secured to said radial portionandlying outside of the ring, and a contact shoe'disposed between said armand the resistance film and slidable on the latter.

4. Avariable resistor comprising a rigid pre formed closed ring ofinsulation material having a cylindrical outer surface, the radialthickness of said ring being large enough to give the ring structuralrigidity yet small enough to leave a substantial space at the center ofthe ring, an interrupted film of resistance material coated directly onsaid cylindrical surface, a control shaft concentric with the ring, acontact arm insulatedly secured to said shaft, said contact armcomprising a radial portion at one face of the ring, and a resilientarcuate portion secured to said radial portion and lying outside of thering, a contact or wiper shoe disposed between said arm and theresistance film, and bearing tangentially against the cylindricalsurface in order to provide line contact therewith, said shoe beingpivoted for transverse oscillation relative to the arm, and

connection means to said arm, said connection means being disposedwithin said ring and including a metallic ring and a wiper ,in contacttherewith, said ring and wiper being insulated from the control shaft.

5. A variable resistor comprising a cylindrical casing with an end wallmade of insulation, a ring of insulation material, said ring having itsperipheral surface coated with a film of resistance material, meanssecuring the ring with its side face tightly against said side wall,soldering lugs having their outer endsproiecting outwardly from thecylindrical wall of the casing and having their inner ends interposedbetween the ring and insulation surface and held in place by thepressure therebetween, and conductive paint "extending from theresistance film around the face of the ring to the lugs.

6. A variable resistor comprising a ring of insulation material, theradial thickness of said ring being small enough to leave a substantialspace at the center of the ring, said ring having a cylindrical outersurface coated with a film of resistance material, means securing thering to a suitable insulation surface, soldering lugs interposed betweenthe ring and'insulation surface, means connecting the resistance film toone of the lugs, a control shaft, a contact arm insulatedly mounted onsaid shaft and supporting a wiper shoe in contact with the resistancefilm, and means within said ring connecting the contact arm to anotherof the lugs.

'7. A variable resistor comprising a ring of insulation material, saidring having acylindrical outer surface coated with an interrupted filmof resistance material, said ring being relatively thick in radialdirection and perforated to receive screws passing therethrough in anaxial direction and securing the ring to a suitable insulation surface,the radial thickness of said ring being small enough to leave asubstantial space at the center of the ring, three soldering lugsinterposed between the ring and insulation surface and held in place bythe ring, conductive paint extending from the ends of the resistancefilm around the face of the ring for connecting.

the ends of the resistance film to the outer two lugs, a control shaftconcentric with the ring,

and a contact arm insulatedly mounted on said" shaft and supporting awiper shoe in yieldable sliding contact with the resistance film, thecenter lug within the ring and bearing against the contact arm in thespace within the ring.

8. A variable resistor comprising a hollow casing open at one end, abushing secured to' the closed end, a control shaft passingtherethrough,

a ring of insulation material having a cylindrical outer surface coatedwith an interrupted film of resistance material, means securing the sameto the closed end of the casing in concentric relation with the shaftand bushing, soldering lugs passing through the casing and having theirinner ends interposed between the ring and casing, means connecting theresistance film to one or more lugs, a contact arm mounted on the innerend of the control shaft and supporting a wiper shoe in contact with theresistance film, means within the ring connecting the contact arm toanother lug, a stop arm mounted on the inner end of the control shaftand holding the contact arm in place, a closure plate outside the stoparm for closing the open end of the casing, and a stop lug on the innerface of said closure plate cooperating with the stop arm.

9. A variable resistor comprising a hollow casing open at one end, abushing secured to the closed end, a control shaft passing therethrough,a ring of insulation material having a cylindrical outer surface coatedwith an interrupted film ing lugs passing radially through the casingand having their inner ends interposed between the ring and casing andheld in place thereby, conductive paint extending from the ends of theresistance film around the face of the ring to two of the lugs, acontact arm mounted on the inner end of the control shaft and supportinga wiper shoe in yieldable contact with the resistance film, the thirdlug coming within the ring and bearing yieldably against a part of thecontact arm, a stop arm mounted on the end of the control shaft andholding the contact arm in place, a closure plate outside the stop armfor closing the open end of the casing, and a stop lug on the inner faceof said closure plate cooperating with the stop arm. g

10. A variable resistor comprising a ring of insulation material havingacylindrical surface and a side face adjoining said cylindrical surface,said cylindrical surface being coated with a film of resistancematerial, a soldering lug bearing against said side face, conductivepaint extending from an intermediate point on the resistance film to theface of the ring and along the face of the ring to the lug, whereby theresistance may be tapp d at a desired intermediate point even ifdisplaced from the lug.

11. A variable resistor comprising a ring of insulation material havinga cylindrical outer surface and a side face, said cylindrical outersurface being coated with an interrupted film of resistance material,means securing the same to an insulation surface with the side facelying against said surface, soldering lugs having their inner endsinterposed between thering and surface, conductive paint extending fromone end of the resistance film around the side face of the ring to oneof the lugs, conductive paint extending from an intermediate point onthe film to the side face of the ring and along the face of the ring toanother lug, whereby the resistance may be tapped at a desiredintermediate point even if displaced from the lug.

12. A variable resistor comprising a ring of insulation material havinga cylindrical outer surface and a side face, said cylindrical outersurface being coated with an interrupted film of resistance material,means securing the same to an insulation surface with the side facelying against said surface, four soldering lugs passing radially throughthe casing and having their inner ends interposed between the ring andsurface, conductive paint extending from the ends of the resistance filmaround the side face of the ring to two of the lugs, a contact armmounted on a control shaft and supporting a wiper shoe in yieldablecontact with the resistance film, a third lug being arranged to bearagainst the contact arm, and conductive paint extending from anintermediate point on the resistance film to the face of the ring andalong the face of the ring to the fourth lug, whereby the resistance maybe tapped at a desired intermediate point by the lug without changingthe position of the lug.

13. A variable resistor comprising a casing, a. bushing secured to oneend of said casing, a control shaft passing therethrough, a ring ofinsulation material, said ring having a cylindrical outer surface coatedwith an interrupted film of resistance material, a contact arm on saidcontrol shaft, said arm comprising an annular hub portion, a radialportion alongside the ring, and an arcuate portion secured to saidradial portion and lying outside of the periphery of the ring, a contactshoe interposed between the end of the arcuate portion and theresistance film, and a soldering lug having a resilient portion bearingagainst the hub portion of the contact arm.

14. A variable resistor comprising a casing, a bushing secured to oneend of said casing, a control shaft passing therethrough, a ring ofinsulation material, said ring having a cylindrical outer surface and aside face, said cylindrical outer surface being coated with aninterrupted film of resistance material, means securing the ring to thecasing, soldering lugs passing through the casing and having their innerends interposed between the ring and casing, conductive paint extendingfrom the resistance film around the side face of the ring to one or morelugs, a contact arm on said control shaft, said arm comprising anannular hub portion, a radial portion, and an arcuate portion secured tosaid radial portion and lying outside of the periphery of the ring, acontact shoe interposed between the end of the arcuate portion and theresistance film, said shoe contacting tangentially with the fdin forline contact and being oscillatable on the arm, one lug passing withinthe ring and having a resilient annular portion bearing against the hubportion of the contact arm.

15. A variable resistor comprising a hollow casing open at one end, abushing secured to the closed end, a control shaft passing therethrough,a ring of insulation material, said ring having a cylindrical outersurface coated with an interrupted film of resistance material, meanssecuring the same to the closed end of the casing in concentric relationwith the shaft and bushing, soldering lugs passing through the casingand having their inner ends interposed between the ring and casing andheld in place thereby, a contact arm on said control shaft, said armcomprising an annular hub portion, a radial portion arranged at the faceof the ring, and an arcuate portion secured to said radial portion andlying outside of the periphery of the ring, a contact shoe interposedbetween the end of the arcuate portion andthe resistance film, a thirdlug having a resilient annular portion bearing against the hub portionof the contact arm, a stop arm on said shaft, a closure plate for theopen end of the casing, and a stop lugon said closure plate cooperatingwith the stop arm.

16. A variable resistor comprising a casing open at one end, a bushingsecured to the closed end, a control shaft passing therethrough, acircular resistance element, a contact arm mounted on the control shaftfor contact with the resistance element, a stop arm mounted on thecontrol shaft, a power switch assembly, a closure for the resistorcasing forming a part of the power switch assembly, a stop lug on theclosure for cooperating with the stop arm to limit the movement ineither direction of the control shaft, and operating means on said powerswitch projecting through the closure into the resistor casing forcooperation with the stop arm of the resistor.

17. A variable resistor comprising a casing open at one end, a bushingsecured to the closed end, a control shaft passing therethrough, a ringof insulation material having a cylindrical outer surface coated with aninterrupted film of resistance material, means securing the same to theclosed end of the casing, a contact arm mounted on the control shaft andSllpIJOltln a wiper shoe in yieldable contact with the resisiance film,a stop arm mounted at the end of the control shaft, a power switch, aclosure or the resistor casing forming a part of the power switcl,assembly, a stop lug on the closure i'J-r cooperaiing with the stop armto limit th :l' the control shaft, and means: switch projecting throughthe closure into the resistor casing for cooperation with the stop armof the resistor.

18. A variable resistor comprising a hollow cas ing open at one end, abushing secured to the closed end, a control shaft passing thercthrougha ring of insulation material having a cylindrical outer surface coatedwith an interrupted film resistance material, means securing the same ti the closed end of the casing in concentric relation with the shaft andbushing, soldering lugs passing radially through the casing and havim:their inner ends interposed between the ring and casing and held inplace thereby, conductive ."vment of said power paint extending from theresistance film around the face of the ring to one of the lugs, acontact arm mounted on the control shaft and suppor ing a wiper shoe inyieldable contact with the re sistance film, one of the lugs beingarranged to bear yieldahly against a part of the contact arm. a stop armmounted on the control shaft, a power switch assembly, a closure discfor the resistor casing forming a part of the power switch assem bly, astop lug on the closure for cooperating with the stop arm to limit themovement of the control shaft, and means on said power switch projectingthrough the closure disc into the resistor casing for cooperation withthe stop arm of the resistor,

' 19. A tandem variable resistance unit comprising a main singleresistance unit of standard type mounted within a casing having an openrear end adapted to detachably receive a closure plate, a

control shaft for varying the resistance of the received within the openend of the first unit, a bushing on said forward end supporting acontrol shaft for the second unit, and means for coupling the controlshaft of the second unit to the control shaft of the first unit when theunits are connected in tandem, the part of said means on the first unitbeing standard to the first unit even when no second unit is added.

20. A tandem resistance unit comprising a main resistance unit ofstandard type mounted within a casing having an open rear end adapted todetachably receive a closure plate having a stop lug, a secondresistance unit mounted within a casing having a similar open rear endadapted to receive said closure plate, the shafts of each of said unitshaving a stop arm for cooperation with the stop lug, the forward end ofsaid second unit being dimensioned like the closure plate and adapted tobe received within the open end of the first unit, and means on thecontrol shaft of the second unit mating with the regulation stop arm ofthe first unit when the units are connected in tandem.

21. A tandem resistance unit comprising first and second resistanceunits arranged end to end in coaxial relation, each of said unitscomprising a hollow casing open at one end, a bushing secured to theclosed end, a shaft passing therethrough, a resistance element, acontact arm mounted on the shaft and supporting a wiper shoe in contactwith the resistance element, and a stop arm mounted on the shaft, theshaft of the first unit projecting well out of the unit for manualoperation, a closure plate adapted to close the open end of eithercasing, a stop lug on said closure plate cooperating with the-stop arm,the closed end of the casing of the second unit being dimensioned likethe closure plate and adapting the same to be received at the open endof the first unit, the shaft of the second unit having at its forwardend a coupling member cooperating with the regular stop arm of the firstunit, so (that both units are operated: in unison.

22. A tandem resistance unit comprising first and second resistanceunits arranged end to end in coaxial relation, each of said unitscomprising a hollow casing open at one end, a bushing secured to theclosed end, a shaft passing therethrough, a resistance means securingthe same to the closed end of the casing, a contact arm mounted on theshaft and supporting a wiper shoe in contact with the resistance film,and a stop arm mounted directly on the end of the shaft, the shaft ofthe first unit projecting well out of the closed end of the unit formanual operation, aclosure plate adapted to close the open end of eithercasing, a stop lug on said closure plate cooperating with the stop arm,the forward end of the casing of the second unit being dimensioned likethe closure plate and adapting the same to be received at the open endof the first unit, the shaft of the second unit having at its forwardend a coupling member cooperating with the regular stop arm of the firstunit,

'so that both units are operated in unison.

23. A variable resistor comprising a hollow casing open at one end, abushing secured to the closed end, a control shaft passing therethrough,an insulation ring smaller in diameter than the casing and coated with aresistance film on its outer cylindrical wall, a contact arm mounted onthe control shaft and having a part lying between the exterior of thering and the interior of the casing, said part extending in a peripheraldirection and being relatively long and yieldably contacting with theresistance ring, a closure I plate closing the open end of the casing, apower switch on said closure plate, means on said shaft to actuate saidswitch, and a single screw for detachably holding the closure plate onthe casing, said screw passing through the casing and ring and beingthreaded into said closure plate at a point substantially inwardly ofthe casing wall and displaced from the center but so positioned as notto interfere with the oscillation of the contact arm over the effectiveportion of the resistance ring, nor with the said switch actuatingmeans.

24. A variable resistor comprising a hollow cup shaped casing open atone end and closed at the other, a bushing permanently secured to theclosed end, a control shaft passing therethrough, the outer end of theshaft projecting from the closed end of the casing, and the inner end ofthe shaft terminating inside the casing, a ring of insulation materialcoated with a film of resistance material, means securing the same tothe closed end of the casing, a contact arm mounted on the control shaftinside the casing and supporting a wiper shoe in yieldable contact withthe resistance film, a stop arm mounted directly on the inner end of thecontrol shaft within the casing, a thin fiat closure plate fully closingthe open end of the casing beyond the stop arm, and a stop lugprojecting inwardly into the casing directly from the inner face of saidclosure plate, said stop lug cooperating with the stop arm to limit themovement of the shaft in both directions.

25. A tandem variable resistance comprising a main resistance unitincluding a casing having an open rear end adapted to detachably receivea closure plate, a second resistance unit including a casing having asimilar open rear end adapted to receive said closure plate, aresistanceelement and a cooperating control shaft and contact arm in each of saidcasings, the forward end of said second unit being closed anddimensioned like the closure plate and adapted to be received within theopen end of the first unit, means for coupling the control shaft of thesecond unit to i the control shaft of the first unit when the.

the effective portions of the resistance elements.

26. A variable resistor comprising a stationary approximatelycylindrical casing, a stationary ring formed of insulating materialdisposed concentrically within said casing and having a cylindricalouter surface, the ring having a radius smaller than the casing toprovide an annular space between the periphery of the ring and theinterior of the casing, said ring being secured to one end of thecasing, a film of graphitic, carbonaceous, or like resistance materialcoated on the periphery of said ring and extending from side edge toside edge of the ring, the band of resistance material being equal inwidth to the thickness of the ring, a control shaft disposed axially ofand rotatable relative to said ring and casing, an arm extendingradially from said shaft outside of that face of the ring opposite theface secured to one end of the casing, a metallic contact shoe engagingsaid film of resistance material and adapted to slide thereover, and aresilient readily yieldable arcuate arm partially surrounding said ringand fixedly secured at one end to the radial arm, said contact shoebeing secured at the other end of the yieldable contact; arm, saidarcuate arm and contact shoe being received in the aforesaid annularspace between the ring and casing.

27. A variable resistor comprising a hollow casing open at one end, abushing secured to the closed end, a control shaft passing therethrough,a ring of insulation material coated with a film of resistance material,means securing the same to the closed end of the casing, a contact armmounted on the control shaft and supporting a wiper shoe in yieldablecontact with the resistance film, a stop arm mounted directly on the endof the control shaft within the casing, and a closure for the open endof the casing, said closure having a stop lug projecting, inwardlydirectly from the inner face of the closure and cooperating with thestop arm, the open end of the casing being adapted to interchangeablydetachably receive said closure in either of two forms, one of saidclosures being in the form of a plain plate and the other being in theform of a power switch, the aforesaid stop lug having an effectivelocation best suited to the partioular'form of closure on which it ismounted.

HENRY G. RICHTER.

